Laser rangefinders determine the distance to a target by emitting a brief, narrow beam light pulse to a target and measuring the time for the reflected light to return. Since the speed of light in air is constant, accurate measurements can be obtained through the use of such devices. Most targets are non-reflective, absorbing some of the light and dispersing the remainder in all directions. As a result, the received light pulse is very faint and decreases as the square of the target distance. For laser rangefinders that emit ‘eye safe’ levels of energy, and have restricted receive lens areas, the received light pulse amplitude from any target over a few hundred yards in range is buried in photo detector noise.
Laser rangefinders typically establish a threshold that is above the noise level, and trigger a timing circuit upon the received pulse exceeding the threshold. Alternately, the threshold is set somewhat lower (into the noise), causing numerous authentic and false triggering ‘hits’, whereupon after several repetitive pulses, a correlation can be established between pulse hit results to establish a ‘most probable’ signal pulse location. This later technique is somewhat effective in improving the ability of the rangefinder to range more distant targets, but is computationally intensive. It can also require a significant amount of memory to be effective.
Furthermore, rangefinders that employ semiconductor laser diodes deliver extremely high current and extremely brief pulses to their laser diodes, while simultaneously supporting extremely sensitive receive circuitry to detect the very small reflected light pulse. Typical laser pulse peak currents can be from 2 to 20 amperes, with durations on the order of 5 to 25 nS. Typical received light pulses from distant targets can be as small as a few hundred photons. Integrating high voltage, high current switching devices along with sensitive receiving circuits into compact units is difficult.
The generation of extremely short, high current pulses is problematic when unavoidable driving component lead inductances are considered. A solution to overcoming the lead inductance problem is to operate the driving circuitry at rather high voltages (20 to several hundred volts), wherein a small capacitor is charged to a high voltage and then discharged with a semiconducting switching device into the laser diode. Such high voltages however, in a portable, battery powered system, are typically developed through the use of a switching power supply that will also generate switching noise that is deleterious to the sensitive receive circuitry.